I have no experience with making a combat scenario but I imagine there is nothing stopping you from making one for Eureka.
Combat Assist doesn’t currently (and perhaps will never?) work with Scenarios so you would need to make an ingame macro that was capable of defeating whatever you’re farming with no other input.
You would also need to create a grid that would allow you to move around the area, and then construct a scenario where you traveled from waypoint to waypoint using your macro for sufficient time to defeat enemies.
Obviously that would be a complicated and messy Scenario that is likely different for every job you play and enemy you fight, so it would be hard to impossible for someone to make one for you!